This application claims the benefit of Korean Patent Application No. 1999-59601, filed on Dec. 21, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to thin film transistors and to liquid crystal display (LCD) devices.
2. Discussion of the Related Art
In general, a LCD includes upper and lower substrates and an interposed liquid crystal layer. The upper substrate includes a color filter and a common electrode, while the lower substrate includes an array of switching elements and a pixel electrode. The liquid crystal layer is comprised of optically anisotropic liquid crystal molecules that arrange according to the state of the switching element.
The lower substrate is often referred to as an array substrate. That substrate is manufactured using various processes such as deposition, photolithography, and etching. Manufacturing the lower substrate involves repeating those processes several times. Of the various processes, the principles of the present invention most directly relate to photolithography. Therefore, photolithography will be explained in more detail.
FIG. 1 is a photolithography flow chart, and FIGS. 2A to 2D are cross-sectional views illustrating a typical photolithography process. In general, photolithography includes deposition, (light) exposure, and development.
First, as shown in FIG. 2A, a thin film 11 is deposited on a glass substrate 10. Referring now to ST100 of FIG. 1, after the deposition of the thin film 11 the glass substrate 10 is pre-baked to remove humidity on the substrate""s surface. This enhances adhesion between the glass substrate 10 and a subsequently deposited photoresist 13. Thereafter, the photoresist 13 is uniformly coated on the thin film 11 using spin-coating. Then, a soft-bake process that evaporates solvents remaining in the photoresist 13 is performed. This hardens the photoresist 13. The result is as illustrated in FIG. 2a. 
ST200 of FIG. 1 is carried out by placing the glass substrate 10 in a light exposure device and aligning the glass substrate with a photomask. This is shown in FIG. 2B, which shows a mask 15. The glass substrate 10 then undergoes light exposure for a predetermined period of time. Beneficially, the light exposure uses ultra-violet (UV) radiation. Due to the light exposure and the mask 15, the portion of the photoresist 13 that is not protected by the mask 15 is polymerized.
After polymerization, ST 300 of FIG. 1 is performed as illustrated by FIG. 2C. Depending on the particular process being used, either the exposed portion or the non-exposed portion of the photoresist 13 is selectively removed by a developer. The result is a photoresist pattern 13. To enhance the adhesion of the photoresist pattern 13 to the thin film 11 the glass substrate 10 is hard-baked by performing a heat-treatment at a predetermined temperature.
After the hard-bake, ST400 (etch) of FIG. 1 is performed as shown in FIG. 2D. Using either a dry or a wet etch process, the thin film 11 is selectively removed with the photoresist 13 acting as a mask. The result is a thin film pattern 11a. In other words, a portion of the thin film 11 that is not covered with the photoresist pattern 13 is removed.
After the etch, ST500 (strip) of FIG. 1 is performed, also as illustrated in FIG. 2D. The photoresist 13 on the thin film pattern 11a is removed using a chemical stripper. After stripping, a cleaning process is carried out, beneficially one that uses distilled water. Since the stripper tends to stick to and solidify on the surface of the substrate 10, the substrate is first immersed in an isopropyl alcohol solution ((CH3)2CHOH) to prevent stripper solidification. Then, the substrate 10 is cleaned using distilled water.
The structure of a conventional array substrate that is manufactured using the foregoing photolithography process is explained below. FIG. 3 is a plan view illustrating an array substrate of a conventional LCD device. As shown in FIG. 3, gate lines 19 are arranged in a transverse direction, and data lines 21 are arranged in a longitudinal direction. Pixel electrodes P are formed at a region defined by the gate and data lines 19 and 21. Thin film transistors (TFTs) are formed at a crossing portion of the gate and data lines 19 and 21. A storage capacitor C is formed adjacent the gate line 19. A portion of the gate line 19 is used as a first electrode of the storage capacitor C, and a portion of the pixel electrode P is used a second electrode. Alternatively, a separate capacitor electrode can be formed and used instead of the portion of the gate line 19.
Each TFT includes a gate electrode 23, an active layer 29, a source electrode 25, and a drain electrode 27 that is spaced apart from the source electrode. The end portion of the active layer 29 is overlapped by the source and drain electrodes 25 and 27.
Each of the foregoing components is formed by photolithography. As the ends of the active layer 29 is overlapped by the source and drain electrodes 25 and 27, the surface of the active layer 29 must be completely clean to prevent problems. Therefore, as described above, the array substrate is immersed in the isopropyl alcohol solution after removing the photoresist using the stripper solution.
However, the isopropyl alcohol solution adversely affects an insulating layer that is beneficially located under the active layer 29. In addition, the isopropyl alcohol solution is relatively costly. Therefore, an improved method of manufacturing a thin film transistor array substrate would be beneficial.
Accordingly, the present invention is directed to a method of manufacturing a thin film transistor array substrate that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
To overcome the problems described above, preferred embodiments of the present invention provide a method of manufacturing a thin film transistor array substrate that includes an effective method of cleaning an active layer of a thin film transistor.
Another object of the present invention is to provide a method of manufacturing a thin film transistor array substrate that includes a low cost process of cleaning an active layer of a thin film transistor.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the above objects, the principles of the present invention provide a method of manufacturing a thin film transistor array substrate that includes forming a gate electrode on a substrate, and then sequentially forming a first insulating layer, a pure semiconductor layer and a doped semiconductor layer that cover the gate electrode. A photoresist layer is then coated on the doped semiconductor layer. The photoresist layer is then exposed and developed to form a photoresist pattern. The pure semiconductor layer and the doped semiconductor layer are then etched to form an active layer and an ohmic contact layer by using the photoresist pattern as a mask. The photoresist pattern is then removed using a stripper. The array substrate is then immersed into a thin alkali-based solution. The array substrate is then cleaned using distilled water. Spaced apart source and drain electrodes are then formed on the ohmic contact layer.
The method further includes forming a second insulating layer over the array substrate, and then forming a pixel electrode on the second insulating layer such that the pixel electrode contacts the drain electrode.
Beneficially, the alkali-based solution is diluted tetramethylamonium hydroxide (TMAH; (CH3)4NOH). The first and second insulating layers are beneficially comprised of inorganic material such as SiNx and SiOx, or of an organic material such as BCB (benzocyclobutene) or an acrylic-based resin. The pixel electrode is beneficially comprised of ITO (indium tin oxide) and/or of IZO (indium zinc oxide). The gate electrode is beneficially comprised of Al, an Al alloy, Mo, Ta, W and/or of Sb. The source and drain electrodes are beneficially comprised of Cr and/or of a chrome alloy.
By using a method of manufacturing a TFT array substrate according to the principles of the present invention, the cleaning of the active layer of a TFT can be improved. In addition, since an alkali-based solution is relatively low cost when compared to an isopropyl alcohol solution, the cost of cleaning the active layer of a thin film transistor can be reduced.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.